A starter circuit for a discharge lamp having preheated electrodes

ABSTRACT

A starter circuit for a discharge lamp includes an inductor in series with the lamp across the AC input terminals and an SCR in shunt with the lamp and in series with the lamp electrodes. A resistor and capacitor are connected in series across the lamp. The impedance of the capacitor is 40 times to 150 times the lampoperating impedance and is approximately 95 percent of the impedance of the series RC circuit. A control circuit ignites the SCR between 30* and 135* after the zero crossing of the AC supply voltage.

United States Patent [72] Inventors Jozef Cornelis Moerkens;

Hilbert Palmers, Emmasingel, Eindhoven, I

Holland [21 Appl. No. 760,451

[22] Filed Sept. 18, 1968 [45] Patented Mar. 9, 1971 73] Assignee U.S. Philips Corporation New York, N.Y.

[32] Priority Sept. 30, 1967 [33] Holland [54] A STARTER CIRCUIT FOR A DISCHARGE LAMP HAVING PREHEATED ELECTRODES 5 Claims, 2 Drawing Figs.

[52] U.S.Cl 315/102, 315/207, 315/208, 315/243 [51] Int. Cl H05b 41/23, H05b 41/392 [50] Field ofSearch 315/98, 101,102, 103, l05,l67,l73,l94, 199, 200, 205, 207,208, 241, 242, 243, 246, 260, 265, 283, 289;

[56] References Cited UNITED STATES PATENTS 3,383,558 5/1968 Waymouth 315/242 Primary Examiner-Roy Lake Assistant ExaminerE. R. LaRoche Attorney-Frank R. Trifari ABSTRACT: A starter circuit for a discharge lamp includes an inductor in series with the lamp across the AC input terminals and an SCR in shunt with the lamp and in series with the lamp electrodes. A resistor and capacitor are connected in series across the lamp. The impedance of the capacitor is 40 times to 150 times the lamp-operating impedance and is approximately- 95 percent of the impedance of the series RC circuit. A control circuit ignites the SCR between 30 and 135 after the zero crossing of the AC supply voltage.

Allll PATENTED MAR 9 WI FIG]- Illl'l' 1N VENTOR. JOZEF C. MOERKENS HILBERT PAL MERS A STARTER CIRCUIT FOR A DlSCRC-E LAMP llAVllNG PREHEATED ELECTRODES The present invention relates to a device for igniting and supplying a gas and/or vapor discharge tube provided with preheated electrodes. This device is adapted to be connected to a sinusoidal source of alternating current source. Two input terminals of the device are shunted by'a series arrangement of the discharge tube and an inductance, the sides of the electrodes remote from the supply are connected together through a semiconductor assembly which passes current in one direction only, and the connections from the electrodes to the semiconductor assembly are substantially free from impedances.

in a known device of the kind described in the preamble, the semiconductor assembly consists of the series arrangement of a breakdown diode and an ordinary diode. A drawback of this known arrangement is that the voltage available for igniting the tube is at most approximately equal to the maximum value of the AC supply voltage. A further drawback is that the resistance in the series arrangement, which forms a shunt of the tube, is still comparatively high also in case this series arrangement carries current, so that the electrode preheating current is limited thereby and in addition undesired heat is produced in the series arrangement.

An object of the invention is to avoid or at least mitigate these drawbacks.

A device according to the invention for igniting and supplying a gas and/or a vapor discharge tube provided with preheated electrodes and adapted to be connected to a sinusoidal alternating current source comprises, two input terminals shunted by a series arrangement of the discharge tube and an inductance, the sides of the electrodes remote from the supply being connected together through a semiconductor assembly which passes current in one direction only and in which the connections from the electrodes to the semiconductor assembly are substantially free from impedances. The device is characterized in that the semiconductor assembly consists of a controlled semiconductor rectifier in which a control circuit triggers the controlled rectifier into conduction, when the device is switched on but the tube is not yet ignited, between 1/126 and %6 seconds after a zero crossing of the AC supply voltage, wherein 6 is the frequency is c/s of the AC supply voltage. The tube is shunted by a series arrangement which comprises at least a capacitor having a capacitive impedance lying between 40 times and 150 times the impedance of the tube in its operating condition, the impedance of the last-mentioned series arrangement being determined mainly by the capacitance.

in this connection the term mainly is to be understood to mean that the capacitor provides at least 95 percent of the impedance of the series arrangement.

An advantage of a device according to the invention is that the voltage available for ignition may be higher than the maximum value of the AC supply voltage. This may be explained as follows. if it is assumed that in a device according to the invention the controlled semiconductor is rendered conducting at an instant which is A6 seconds after a zero crossing of the AC supply voltage, a current starts to flow through the series arrangement of the inductance, the two electrodes of the discharge tube and the controlled rectifier (if at least the direction of the current through the controlled rectifier corresponds to the polarity of the supply voltage). A small current of course also flows through the capacitor shunting the tube. The last-mentioned current is, however, negligibly small due to the comparatively high impedance of this capacitor. Since the electrodes of the discharge tube have only a small resistance, conduction of the controlled rectifier actually only means connecting the inductor to an AC voltage source. If the inductor is switched on at the instant already indicated, namely seconds after the zero-crossing of the AC supply voltage, the current will become zero approximately %6 seconds after this zero-crossing of the AC supply voltage. The current will then attempt to reverse its direction but this is inhibited by the controlled semiconductor rectifier. This actually means that the circuit including the semiconductor rectifier is switched off. in other words, the short circuit across the capacitor, which shunts the tube, is eliminated. This capacitor initially had no charge because the controlled rectifier was conducting. Now, however, the supply voltage has on the contrary a maximum value at the instant that the controlled semiconductor rectifier becomes cut off, thus at the instant he seconds after the zero-crossing of the AC supply voltage. The results is that the capacitor is charged to approximately twice the maximum value of the supply voltage. Said voltage is now also set up across the ends of the discharge tube so as to facilitate the ignition of the tube.

If the controlled rectifier was triggered into conduction at a slightly earlier instant, for example, a seconds after the zerocrossing of the AC supply voltage, where a is smaller than &6, the current flowing through the controlled rectifier would be cut off only at an instant (1/6- a) seconds after the said zerocrossing of the AC supply voltage.

This is because the resistance in the circuit is so small relative to 256 L (wherein L is the magnitude of the self-inductance in series with the discharge tube). The smaller the value of a the more 1/0- a) approaches 1/0, that is to say, the more the instant of the controlled semiconductor rectifier becoming cut off approaches a subsequent corresponding zero-crossing of the AC ,supply voltage, and hence also the lower the instantaneous supply voltage at the instant of the controlled semiconductor rectifier becoming cut off, that is to say, the lower the voltage peak across the capacitor.

If the controlled rectifier is cut off 30 prior to a zerocrossing of the supply voltage, (that is to say, a 1/120) the instantaneous supply voltage is exactly equal to half the peak value of the supply voltage (namely sine 30 /9. It was stated in the foregoing that when the controlled rectifier is cut off the voltage across the capacitor rises to approximately twice the instantaneous value of the supply voltage. If a 1/120 the voltage thus rises to twice the half peak value, or in other words to the peak value of the supply voltage. For producing an ignition voltage which is higher than the peak value of the supply voltage, a thus must be higher than 1/ seconds.

An upper limit for a can be derived in a corresponding manner, thus at the upper limit a 5/ 120. Due to losses it appears, however, that the more practical upper limit is given by a %6.

From the foregoing it follows that in a device according to the invention, in which the instant of ignition of the controlled rectifier lies between 1/120 seconds and %0 seconds after a zero-crossing of the AC supply voltage, the voltage across the discharge tube can be raised to a value higher than the maximum value of the supply voltage.

As a rule the device will be connected to the AC supply voltage at a random instant. A transition phenomenon will then usually occur during a few periods of the AC voltage before the controlled rectifier is rendered conducting within the time interval specified above.

Since the resistance of a controlled semiconductor rectifier, when conducting, is very small, very little heat is produced in the series arrangement which connects the sides of the electrodes remote from the supply.

It will also be evident from the foregoing description that the duration of the electrode preheating current can be ad-' justed by using a controlled element as a starter. it is thus possible to make a compromise between the available voltage on the one hand and preheating current on the other hand, which is advantageous for the ignition of the tube. it is generally preferred to choose the instant of making the controlled rectifier conduct prior to Mid seconds after the zerocrossing of the AC supply voltage. in fact, the electrode current will then be able to flow for a comparatively long period (per cycle of theAC supply voltage) so that these electrodes can be heated quickly. 4

The impedance of the capacitor, which is provided in the series arrangement shunting the tube, must lie within two limits. In fact, said impedance must be smaller than a given multiple of the impedance of the tube, namely smaller than 150 times the impedance of the tube in its operating condition. This is necessary not only to obtain a very short-lasting high voltage across the tube during the starting process, but a voltage which is both high and subsists across the tube for a slightly longer period of time. In fact the tube then ignites with greater reliability. A lower limit is also necessary for the impedance of the capacitor. In fact, the impedance of the capacitor must not be smaller than approximately 40 times the operating of the tube. The last-mentioned requirement is necessary to prevent the waveform of the current from deviating too much from the sinusoidal form during operation of the tube because this may cause a decrease in the lifetime of the tube. A further reason why the impedance of the capacitor must not be too low resides in the fact that otherwise the described switch-on phenomenon of the inductor will not proceed as described in the foregoing.

True, it is known per se to use a heating circuit including a controlled semiconductor switching element for supplying and igniting a tube provided with preheated electrodes. In this case the element is a transistor. In this known case, however, impedances are also incorporated in the heating circuit. The inevitable losses in these impedances limit the preheating current and this is disadvantageous.

The series arrangement including the capacitor can be connected to the sides of the electrodes of the discharge tube which face the supply.

In a device according to the invention, the series arrangement including the capacitor is preferably connected to the sides of the electrodes of the discharge tube remote from the supply, said series arrangement consisting of the capacitor and a resistor.

An advantage of this preferred solution is that all elements for starting the discharge tube can be combined in one auxiliary device which can be connected to the tube by means of only two connecting terminals.

The resistor in series with the capacitor serves to limit the capacitor current flowing through the controlled rectifier.

If, as already stated, the capacitor is connected to the sides of the tube electrodes facing the side of the supply, it is true that the capacitor current flowing through the controlled semiconductor rectifier is limited by the resistance of these electrodes of the discharge tube so that no additional auxiliary resistor is required, but this advantage is offset by the fact that the starting device can no longer be connected in a simple manner by means of two connections only. A second disadvantage is that in the case of a faulty tube or a tube which has been removed from its holders, a current continues to flow through the capacitor. Said disadvantages do not exist in the preferred solution mentioned above.

In order that the invention may be readily carried into effect, it will now be described in detail, by way of example, with reference to the accompanying diagrammatic drawing, in which:

FIG. 1 is a diagram of a device according to the invention; and

FIG. 2 is a graph in which some voltages of the device of FIG. 1 are plotted against time.

In FIG. 1 the input terminals ofa device according to the invention are indicated by reference numerals 1 and 2. These terminals are intended to be connected to an AC supply of 220 Volt, 50 c/s. The terminals 1 and 2 are shunted by the series arrangement of an inductance 3 and a low-pressure mercury vapor discharge lamp 4. In this embodiment, inductor 3 is a coil. Of course, the inductance may, in certain cases, also be formed by a leakage transformer. The lamp 4 is provided with preheated electrodes 5 and 6. The sides of the electrodes 5 and 6 which are remote from the terminals 1 and 2 are connected together by a series arrangement of a resistor 7 and a capacitor 8. The said sides of the electrodes 5 and 6 are also connected together through a controlled semiconductor rectifier (thyristor) 9. The controlled semiconductor rectifier is provided with a control circuit. This control circuit comprises, inter alia, two resistors 10 and 11 which are connected in series with each other and shunt the main electrodes of the thyristor 9. The junction of the resistors 10 and 11 is connected through a breakdown element (diac 12) to the control electrode of the thyristor 9. The junction of the resistors 10 and 11 is furthermore connected through capacitor 13 to the cathode of the thyristor 9. Finally, the junction between the threshold element 12 and the control electrode of the thyristor 9 is connected through a resistor 14 to the cathode of the thyristor 9.

The tube 4 in the described circuit arrangement is ignited as follows. When the terminals 1 and 2 are connected to the indicated supply source, the auxiliary capacitor 13 in the control circuit of the thyristor 9 is charged through the inductor 3 and the resistor 10. When the capacitor 13 has reached the breakdown value of the threshold element 12, the thyristor 9 is rendered conducting. Then a current flows in the series arrangement formed' by the inductor 3, the electrode 5, the thyristor 9 and the electrode 6. When the current flowing through the thyristor 9 becomes zero, this switching element is cut off. The capacitor 8 is now quickly charged up to approximately double the value of the instantaneous supply voltage. If thereupon the lamp is not ignited, the thyristor 9 is rendered conducting again shortly thereafter. Then the electrode preheating current starts to flow again, etc. This is repeated until the lamp 4 is ignited. As a result the voltage across the lamp decreases to so low a value that a voltage is set up across the resistor 11, hence also across the capacitor 13, at which voltage the element 12 no longer breaks down. In other words, the thyristor 9 is then no longer rendered conducting.

In a concrete case the inductor 3 had a value of 1.2 Henry. The tube 4 was proportioned for 40 Watt. The operating voltage was 103 V. In the operating condition of this tube the tube current was approximately 0.44 amp. The capacitor 8 was 0.15 ,uF. The resistor 7 was 30 Ohms, the resistor 10 was kOhms, the resistor 11 was 20 kOhms and the capacitor 13 was 5 6 kpF, breakdown element 12 had a breakdown value of 32 V and a holding value of 27 V and the resistor 14 was 200 Ohms. In this concrete case the impedance of the capacitor 8 (0.15 ,uF) thus corresponded to approximately 21200 Ohms. The internal ,impedance of the tube 4 was approximately 103/044 the operating condition, which is approximately 240 Ohms. The impedance of the capacitor (for 50 c/s) divided by the impedance of the tube therefore was 21200/240, or approximately 90. In this concrete case a peak voltage of approximately 550 Volt was developed across the tube.

In FIG. 2 the mains voltage E, (voltage across 1,2 of FIG. 1) is shown as a function of time. A zero-crossing of this supply voltage is indicated by t,,. Said supply voltage has a maximum value at an instant t which lies 416 seconds from t Just before 2 that is to say at the instant t,, the thyristor 9 (see FIG. 1) is rendered conducting. The current flowing through the thyristor 9 is indicated by i in FIG. 2. Said current becomes zero at the instant t At this instant a voltage E (500 V) is set up across the tube as a result of the quick charging of the capacitor 8 (see FIG. 1). The said voltage rises (see fig. 2) until the supply voltage is again set up across the tube. FIG. 2 shows that the arrangement of FIG. 1 provides both a longlasting preheating current i, which lasts longer than half a period,

and a high ignition voltage (E,,).

The starter device of the described embodiment comprising the elements 7 up to and including 14 (see FIG. 1) may in the given case be accommodated in a space of 2'/z 3Vz 4 /zcm. Said starter device may be constructed as a separate structural unit. The space occupied by it is not much larger than the space required for a glow discharge starter-switch. The described electronic starter is furthermore considerably less vulnerable than a glow discharge starter-switch.

We claim:

1. A starter circuit for a discharge tube having preheated electrodes comprising, a pair of input terminals adapted for connection to a sinusoidal source of AC voltage, an inductance, means serially connecting said inductance and said discharge tube across said input terminals, a semiconductor controlled rectifier connected directly in shunt with the tube and in series with the tube electrodes, a control circuit coupled to the control electrode of the controlled rectifier and including means for delaying the start of conduction therein for a time period between H120 and %0 seconds after a zero crossing of the AC supply voltage, wherein 0 is the frequency in cycles per second of the AC supply voltage, a capacitor and an impedance element serially connected in shunt with the tube, the capacitance of said capacitor being chosen so that the impedance thereof lies between 40 times and 150 times the operating impedance of the tube, the total impedance of the last-mentioned series arrangement being determined mainly by the capacitor.

2. A starter circuit as claimed in claim 1 wherein said impedance element is a resistor and the series arrangement of the capacitor and resistor is connected to the ends of the tube electrodes that are remote from the input terminals.

3. A starter circuit as claimed in claim 1 wherein the control circuit is connected in shunt with the tube so that the control circuit is responsive to the decrease in voltage across the tube subsequent to ignition thereof to hold the control electrode of the controlled rectifier at a voltage level below the rectifier firing voltage.

4. A starter circuit as claimed in claim 3 wherein said control circuit comprises, a resistor and capacitor connected in series across the tube, and a voltage breakdown element connected between the capacitor and the control electrode of the controlled rectifier.

5. A starter circuit for the discharge tube having a pair of preheated electrodes comprising, first and second input terminals adapted for connection to a sinusoidal source of AC voltage, an inductance connected between the first input terminal and one terminal of a first one of the tube electrodes, means connecting the second input terminal to a first terminal of the second tube electrode, a resistor and a capacitor connected in series between a second terminal of the first electrode and a second terminal of the second electrode, a semiconductor controlled rectifier connected between said second terminals of the first and second electrodes, and a control circuit having first and second input terminals connected between said second terminals of the first and second elec-' trodes and an output terminal connected to the control electrode of the controlled rectifier, said control circuit including means for delaying ignition of the controlled rectifier to a firing angle lying between 30 and after a zero crossing of the AC supply voltage, the capacitance of said capacitor being chosen so that the impedance thereof lies between 40-times and times the operating impedance of the tube, and the impedance of the capacitor being approximately 95 percent of the total impedance of the series arrangement of the resistor and the capacitor. 

1. A starter circuit for a discharge tube having preheated electrodes comprising, a pair of input terminals adapted for connection to a sinusoidal source of AC voltage, an inductance, means serially connecting said inductance and said discharge tube across said input terminals, a semiconductor controlled rectifier connected directly in shunt with the tube and in series with the tube electrodes, a control circuit coupled to the control electrode of the controlled rectifier and including means for delaying the start of conduction therein for a time period between 1/12 theta and 3/8 theta seconds after a zero crossing of the AC supply voltage, wherein theta is the frequency in cycles per second of the AC supply voltage, a capacitor and an impedance element serially connected in shunt with the tube, the capacitance of said capacitor being chosen so that the impedance thereof lies between 40 times and 150 times the operating impedance of the tube, the total impedance of the last-mentioned series arrangement being determined mainly by the capacitor.
 2. A starter circuit as claimed in claim 1 wherein said impedance element is a resistor and the series arrangement of the capacitor and resistor is connected to the ends of the tube electrodes that are remote from the input terminals.
 3. A starter circuit as claimed in claim 1 wherein the control circuit is connected in Shunt with the tube so that the control circuit is responsive to the decrease in voltage across the tube subsequent to ignition thereof to hold the control electrode of the controlled rectifier at a voltage level below the rectifier firing voltage.
 4. A starter circuit as claimed in claim 3 wherein said control circuit comprises, a resistor and capacitor connected in series across the tube, and a voltage breakdown element connected between the capacitor and the control electrode of the controlled rectifier.
 5. A starter circuit for the discharge tube having a pair of preheated electrodes comprising, first and second input terminals adapted for connection to a sinusoidal source of AC voltage, an inductance connected between the first input terminal and one terminal of a first one of the tube electrodes, means connecting the second input terminal to a first terminal of the second tube electrode, a resistor and a capacitor connected in series between a second terminal of the first electrode and a second terminal of the second electrode, a semiconductor controlled rectifier connected between said second terminals of the first and second electrodes, and a control circuit having first and second input terminals connected between said second terminals of the first and second electrodes and an output terminal connected to the control electrode of the controlled rectifier, said control circuit including means for delaying ignition of the controlled rectifier to a firing angle lying between 30* and 135* after a zero crossing of the AC supply voltage, the capacitance of said capacitor being chosen so that the impedance thereof lies between 40 times and 150 times the operating impedance of the tube, and the impedance of the capacitor being approximately 95 percent of the total impedance of the series arrangement of the resistor and the capacitor. 